Mobile circuit switched (CS) services based on Global System for Mobile communications (GSM) and Wideband Code Division Multiple Access (WCDMA) radio access networks (RANs) provide telecommunication services with a single subscription in almost all countries of the world. Today, the number of CS subscribers is still growing due to the provision of mobile CS services in countries, such as India and China, and due to the evolution of the mobile switching center (MSC) architecture into a softswitch solution that permits using packet transport infrastructure for mobile CS services.
The 3rd Generation Partnership Project (3GPP) has completed work (e.g., associated with UMTS Terrestrial Radio Access Networks (UTRANs) and Evolved UTRANs (E-UTRANs)) that defines a Long Term Evolution (LTE) concept that assures competitiveness of 3GPP-based radio access network technology. In parallel with the LTE concept, the 3GPP has also completed work associated with System Architecture Evolution (SAE) core networks. Only a packet switched (PS) domain (e.g., via which all services are supported) has been specified for LTE and SAE networks. However, GSM and WCDMA networks provide both PS and CS access simultaneously.
If telephony services are deployed over a LTE radio access network, an Internet Protocol (IP) Multimedia Subsystem (IMS)-based service engine is required. It has been investigated how to use LTE/SAE as access networks for existing CS core networks. The solutions to the investigation are referred to as “CS over LTE” solutions. The “CS over LTE” solutions provide a Packet MSC (PMSC) (e.g., in the CS core network) that serves both 2G and 3G radio access networks. The PMSC includes two new logical functions: (1) a Packet CS Controller (PCSC) that is used for control plane signaling; and (2) an Interworking Unit (IWU) that is used for user plane signaling. In the “CS over LTE” solutions, communication between user equipment and the PMSC is based on a SGi interface. This means that all direct communication between the user equipment and the PCSC and the IWU (e.g., provided in the PMSC) is based on IP protocols and that the user equipment is visible and reachable using an IP-address (e.g., via a packet data network (PDN) gateway (GW)).
However, none of the “CS over LTE” solutions have been standardized by the 3GPP. Instead, a solution called “CS fallback” has been standardized by the 3GPP. The main principle of “CS fallback” is that the user equipment is performing normal SAE mobility management procedures towards a mobility management entity (MME) while camping on a LTE radio access network. The MME registers the user equipment in a MSC server for CS based services using a SGs interface. When a request for CS services is received by the MSC server, the MSC server forwards the request to the user equipment via the MME (e.g., using the SGs interface) and the user equipment performs a CS fallback to a GSM EDGE Radio Access Network (GERAN) or a UTRAN and responds to request via the selected RAN. Similar behavior applies for mobile originated CS services. When mobile originated CS services are triggered and the user equipment is camping on a LTE radio access network (e.g., an E-UTRAN), the user equipment performs a CS fallback to a GERAN or a UTRAN and triggers initiation of the CS service via the selected RAN. Different solutions have been proposed for the “CS fallback” mechanism, such as a PS handover and an inter-RAN cell change order. Once the user equipment is finished with the CS service, the user equipment returns back to the LTE radio access network (e.g., to the E-UTRAN) using existing mechanisms. Unfortunately, the existing mechanisms used to move the user equipment back to the E-UTRAN (e.g., after CS fallback) are too slow, which may result in a bad experience for a user of the user equipment (e.g., poor transmission quality, dropped calls, etc.).